Optical pickup device and optical disc device

ABSTRACT

An optical pickup device imparts different astigmatisms from each other to light fluxes in four light flux areas A through D formed around an optical axis of laser light, out of the laser light reflected on a disc. The optical pickup device also changes the propagating directions of the light fluxes in the light flux areas A through D to separate the light fluxes in the light flux areas A through D from each other. A signal light area where only signal light exists is defined on a detection surface of a photodetector. Sensing portions are arranged at a position corresponding to the signal light area. Accordingly, only the signal light is received by the sensing portions, thereby suppressing deterioration of a detection signal resulting from stray light.

This application claims priority under 35 U.S.C. Section 119 of JapanesePatent Application No. 2009-102440 filed Apr. 20, 2009, entitled“OPTICAL PICKUP DEVICE AND OPTICAL DISC DEVICE” and Japanese PatentApplication No. 2009-109238 filed Apr. 28, 2009, entitled “OPTICALPICKUP DEVICE AND OPTICAL DISC DEVICE”. The disclosures of the aboveapplications are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical pickup device and an opticaldisc device, and more particularly, relates to an optical pickup deviceand optical disc device suitable in use for recording to and reproducingfrom a recording medium having laminated recording layers.

2. Disclosure of Related Art

In recent years, as the capacity of an optical disc has been increased,an optical disc having an increased number of recording layers has beendeveloped. Laminating recording layers in a disc enables to considerablyincrease the data capacity of the disc. In the case where recordinglayers are laminated, generally, two recording layers are laminated onone side of a disc. Recently, however, laminating three or morerecording layers on one side of a disc has been put into practice tofurther increase the capacity of the disc. Thus, the capacity of a disccan be increased by increasing the number of recording layers to belaminated. However, as the number of recording layers to be laminated isincreased, the distance between the recording layers is decreased, andsignal deterioration resulting from an interlayer crosstalk isincreased.

As the number of recording layers to be laminated is increased,reflection light from a recording layer (a targeted recording layer) tobe recorded/reproduced is reduced. As a result, if unwanted reflectionlight (stray light) is entered into a photodetector from a recordinglayer on or under the targeted recording layer, a detection signal maybe deteriorated, which may adversely affect focus servo control andtracking servo control. In view of this, in the case where a largenumber of recording layers are laminated, it is necessary to properlyremove stray light, and stabilize a signal from a photodetector.

As a method for removing stray light, there is proposed a method using apinhole. In this method, a pinhole is formed at a position where signallight is converged. In this method, an unwanted stray light componententered into a photodetector can be reduced, because a part of straylight is blocked by the pinhole. There is proposed a method using a halfwavelength plate and a polarizing optical element in combination, asanother method for removing stray light. In this method, a polarizationdirection of stray light is changed by the half wavelength plate, andthe stray light is blocked by the polarizing optical element. Thisenables to prevent an unwanted stray light component from being enteredinto a photodetector.

However, in the method for removing stray light using a pinhole, it isnecessary to accurately position the pinhole at a position where laserlight (signal light) reflected on a targeted recording layer isconverged. In this method, therefore, it is difficult to adjust theposition of the pinhole. If the size of the pinhole is increased toeasily adjust the position of the pinhole, stray light is more likely topass through the pinhole, which obstructs the effect of suppressingsignal deterioration resulting from stray light.

In the method for removing stray light by combined use of a halfwavelength plate and a polarizing optical element, each two halfwavelength plates and polarizing optical elements are necessary. Inaddition, two lenses are necessary to remove stray light. Thus, thenumber of parts and the cost are increased. Further, it is cumbersome toadjust the arrangement positions of these members. Furthermore, it isnecessary to secure a space for arranging these members side by side,which may increase the size of an optical system.

SUMMARY OF THE INVENTION

A first aspect of the invention is directed to an optical pickup devicefor irradiating laser light onto a recording medium having a pluralityof recording layers. The optical pickup device according to the firstaspect includes a light source which emits laser light, an objectivelens which converges the laser light on the recording medium, anastigmatism portion into which the laser light reflected on therecording medium is entered; a light flux separating portion, and aphotodetector. The astigmatism portion is provided with a plurality oflens areas formed around an optical axis of the laser light. Theastigmatism portion imparts astigmatism to the laser light individuallywith respect to each of the lens areas. The light flux separatingportion makes propagating directions of a light flux of the laser lightto be entered into each of the lens areas different from each other toseparate the light fluxes from each other. The photodetector receivesthe separated light fluxes to output a detection signal.

A second aspect of the invention is directed to an optical disc device.The optical disc device according to the second aspect includes theoptical pickup device according to the first aspect, and a computingcircuit which processes an output from the photodetector.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects, and novel features of the present inventionwill become more apparent upon reading the following detaileddescription of the embodiment along with the accompanying drawings.

FIGS. 1A through 1C are diagrams for describing a technical principle (aconvergence state of light rays) in an embodiment of the invention.

FIGS. 2A through 2H are diagrams for describing the technical principle(distribution states of light fluxes) in the embodiment.

FIGS. 3A through 3D are diagrams for describing the technical principle(distribution states of signal light and stray light) in the embodiment.

FIGS. 4A through 4D are diagrams for describing the technical principle(distribution states of signal light and stray light) in the embodiment.

FIGS. 5A and 5B are diagrams for describing the technical principle (amethod for separating light fluxes) in the embodiment.

FIGS. 6A through 6D are diagrams showing a method for arranging sensingportions in the embodiment.

FIG. 7 is a diagram showing an optical system of an optical pickupdevice as an example in the invention.

FIGS. 8A and 8B are diagrams showing a construction example of ananamorphic lens in the inventive example.

FIGS. 9A and 9B are diagrams showing a distribution state of signallight and stray light, in the case where a disc has two recording layersin the inventive example.

FIGS. 10A through 10D are diagrams showing a method for adjusting thepositions of an anamorphic lens and a photodetector in the inventiveexample.

FIGS. 11A and 11B are diagrams for describing a distribution of lightfluxes in modification example 1.

FIGS. 12A and 12B are diagrams for describing a method for arrangingsensing portions in modification example 1.

FIGS. 13A through 13C are diagrams for describing a focus error signalin modification example 1.

FIGS. 14A and 14B are diagrams for describing a distribution of lightfluxes in modification example 2.

FIGS. 15A and 15B are diagrams for describing a method for arrangingsensing portions in modification example 2.

FIGS. 16A and 16B are diagrams for describing a focus error signal inmodification example 2.

FIGS. 17A through 17C are diagrams showing another modification exampleof the inventive example.

The drawings are provided mainly for describing the present invention,and do not limit the scope of the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS In the following, an embodiment ofthe invention is described referring to the drawings. TechnicalPrinciple

First, a technical principle which is applied to an embodiment of theinvention is described referring to FIGS. 1A through 6D.

FIGS. 1A through 1C are diagrams showing a convergence state of lightrays. FIG. 1A is a diagram showing a convergence state of laser light(signal light) reflected on a targeted recording layer, laser light(stray light 1) reflected on a layer away from a laser light incidentside than the targeted recording layer, and laser light (stray light 2)reflected on a layer closer to the laser light incident side than thetargeted recording layer. FIG. 1B is a diagram showing an arrangement ofan anamorphic lens to be used in the principle of the embodiment. FIG.1C is a diagram showing an arrangement of an anamorphic lens to be usedin a focus adjusting method based on a conventional astigmatism method.

Referring to FIG. 1C, the anamorphic lens to be used in the conventionalmethod converges laser light to be entered into the anamorphic lensparallel to an optical axis of the lens in a curved surface directionand a flat surface direction. In this embodiment, the curved surfacedirection and the flat surface direction are orthogonal to each other.Further, the curvature radius in the curved surface direction is smallerthan the curvature radius in the flat surface direction, andaccordingly, the effect of converging laser light to be entered into theanamorphic lens is larger in the curved surface direction than in theflat surface direction. To simplify the description on the astigmatismfunction of the anamorphic lens, in this embodiment, the terms “curvedsurface direction” and “flat surface direction” are used. Actually,however, the shape of the anamorphic lens in the “flat surfacedirection” in FIG. 1C is not limited to a flat shape, as far as theanamorphic lens is operable to form focal lines at different positionsfrom each other. In the case where laser light is entered into theanamorphic lens in a convergence state, the shape of the anamorphic lensin the “flat surface direction” may be a linear shape (where thecurvature radius=∞).

The anamorphic lens to be used in the principle of the embodiment isdifferent from the anamorphic lens shown in FIG. 1C in the arrangementin the following points. Specifically, the anamorphic lens shown in FIG.1C is divided into four areas A through D by two straight lines parallelto the flat surface direction and the curved surface direction. As shownin FIG. 1B, the anamorphic lens to be used in the principle of theembodiment is configured in such a manner that the lens configurationsi.e. the lens curved surfaces of areas A and D among the four areas Athrough D are replaced with each other in the curved surface directionand the flat surface direction. The lens configurations of the areas Band C in FIG. 1B are the same as those of the areas B and C of theanamorphic lens shown in FIG. 1C.

The anamorphic lens to be used in the principle of the embodiment mayhave arrangements as shown in e.g. FIGS. 11A and 14A, other than thearrangement shown in FIG. 1B. The anamorphic lenses shown in FIGS. 11Aand 14A will be described later in detail.

Referring to FIG. 1A, signal light converged by the anamorphic lensforms focal lines at different positions from each other by convergencein the curved surface direction and the flat surface direction. Thefocal line position (S1) by convergence in the curved surface directionis closer to the anamorphic lens than the focal line position (S2) byconvergence in the flat surface direction. The convergence position (S0)of signal light to be described later is an intermediate positionbetween the focal line positions (S1) and (S2) by convergence in thecurved surface direction and the flat surface direction.

Similarly to the above, the focal line position (M11) by convergence ofstray light 1 by the anamorphic lens in the curved surface direction iscloser to the anamorphic lens than the focal line position (M12) byconvergence of stray light 1 in the flat surface direction. In view ofthe above, the anamorphic lens is designed in such a manner that thefocal line position (M12) by convergence of stray light 1 in the flatsurface direction is to the anamorphic lens than the focal line position(S1) by convergence of signal light in the curved surface direction.

Similarly to the above, the focal line position (M21) by convergence ofstray light 2 in the curved surface direction is closer to theanamorphic lens than the focal line position (M22) by convergence ofstray light 2 in the flat surface direction. In view of the above, theanamorphic lens is further designed in such a manner that the focal lineposition (M21) by convergence of stray light 2 in the curved surfacedirection is away from the anamorphic lens than the focal line position(S2) by convergence of signal light in the flat surface direction.

A beam spot of signal light becomes a least circle of confusion at theconvergence position (S0) between the focal line position (S1) and thefocal line position (S2).

FIGS. 2A through 2H are diagrams showing distribution states of signallight on respective observation planes perpendicular to an optical axisof laser light. The terms “area A” through “area D” in FIGS. 2A through2H denote light flux areas of signal light to be entered into the areasA through D of the anamorphic lens shown in FIG. 1B. In FIG. 2A through2H, the terms “curved” and “flat” respectively denote “curved surfacedirection” and “flat surface direction” in the areas A through D of theanamorphic lens.

FIGS. 2A, 2C, 2E, and 2G are diagrams respectively showing distributionstates of signal light, in the case where the observation planes arelocated at the position of the anamorphic lens, the focal line position(S1), the convergence position (S0), and the focal line position (S2).Further, FIGS. 2B, 2D, and 2F are diagrams respectively showingdistribution states of signal light, in the case where the observationplanes are located at a position between the anamorphic lens and thefocal line position (S1), a position between the focal line position(S1) and the convergence position (S0), and a position between theconvergence position (S0) and the focal line position (S2). FIG. 2H is adiagram showing a distribution state of signal light, in the case wherethe observation plane is located at a position away from the anamorphiclens than the focal line position (S2).

Light in each of the light flux areas shown in FIG. 2A is subjected toconvergence in the curved surface direction and the flat surfacedirection by a lens portion corresponding to each of the areas of theanamorphic lens. As described above, since the convergence function inthe curved surface direction is larger than the convergence function inthe flat surface direction, the shapes of light in each of the lightflux areas are changed resulting from the difference in the convergencefunction, as shown in FIGS. 2B through 2H, as the light is propagated.

As shown in FIGS. 2C and 2G, light in each of the light flux areas has alinear shape (forms a focal line) at the focal line positions (S1) and(S2). Further, as shown in FIG. 2E, signal light has a circular shape(forms a least circle of confusion) at the convergence position (S0).Furthermore, as shown in FIG. 2D, after passing the focal line position(S1), light in each of the light flux areas partly enters into anadjacent area crossing over one of two parting lines which divides thetotal light flux into four. Furthermore, after passing the focal lineposition (S2), as the light is propagated, light in each of the lightflux areas is changed in such a manner that the size of thecorresponding light flux area is increased.

In FIGS. 2A through 2H, only the distribution states of signal light areshown. Similarly to signal light, distribution states of stray light 1and stray light 2 are changed depending on a positional relation betweenthe observation planes, and the focal line positions defined byconvergence of light in the curved surface direction and the flatsurface direction.

Next, a relation between irradiation areas of signal light and straylight 1 and 2 on a plane S0 is described, considering the above.

As shown in FIG. 3A, the anamorphic lens is divided into four areas Athrough D. The shapes of reflection light (signal light, and stray light1 and 2) from a disc are changed as described above referring to FIGS.2A through 2H by convergence of the anamorphic lens. As described above,signal light in the light flux areas A through D is distributed on theplane S0 as shown in FIGS. 3B. Further, since stray light 1 has anirradiation state on the plane S0 as shown in FIG. 2H, stray light 1 inthe light flux areas A through D is distributed on the plane S0 as shownin FIG. 3C. Since stray light 2 has an irradiation state on the plane S0as shown in FIG. 2B, stray light 2 in the light flux areas A through Dis distributed on the plane S0 as shown in FIG. 3D.

In this embodiment, if signal light, and stray light 1 and 2 areextracted on the plane S0 with respect to each of the light flux areas,the distributions of signal light, and stray light 1 and 2 are as shownin FIGS. 4A through 4D. In this case, signal light in each of the lightflux areas is not superimposed on any of stray light 1 and 2 in thecorresponding light flux area. Accordingly, separating the light fluxes(signal light, and stray light 1 and 2) in each of the light flux areasin different directions from each other, and then allowing only signallight to be received by a corresponding sensing portion enables torealize incidence of only signal light on the corresponding sensingportion, while preventing incidence of stray light. This enables toavoid deterioration of a detection signal resulting from stray light.

As described above, dispersing light passing the areas A through D, andseparating the light on the plane S0 from each other enables to extractonly signal light. The embodiment is made based on the above principle.

FIGS. 5A and 5B are diagrams showing distribution states of signallight, and stray light 1 and 2 on the plane S0, in the case where thepropagating directions of light fluxes (signal light, and stray light 1and 2) passing the four areas A through D shown in FIG. 3A are changedby a certain angle in different directions from each other. FIG. 5A is adiagram of the anamorphic lens, when viewed in the optical axisdirection (the propagating direction of laser light at the time ofincidence into the anamorphic lens) of the anamorphic lens, and FIG. 5Bis a diagram showing distribution states of signal light, and straylight 1 and 2 on the plane S0.

In FIG. 5A, the propagating directions of light fluxes (signal light,and stray light 1 and 2) passing the areas A through D are respectivelychanged into directions Da, Db, Dc, and Dd with respect to thepropagating directions of the respective light fluxes before incidenceby a certain angle amount α (not shown). The directions Da, Db, Dc, andDd are respectively inclined by an angle of 45° with respect toboundaries between the areas A and B, the areas B and C, the areas C andD, and the areas D and A.

In the above arrangement, signal light, and stray light 1 and 2 in eachof the light flux areas can be distributed on the plane S0, as shown inFIG. 5B, by adjusting the angle amount α with respect to the directionsDa, Db, Dc, and Dd. As a result, as shown in FIG. 5B, a signal lightarea where only signal light exists can be defined on the plane S0.Arranging sensing portions of a photodetector at a positioncorresponding to the signal light area allows only signal light in eachof the areas to be received by the corresponding sensing portions of thephotodetector.

FIGS. 6A through 6D are diagrams for describing a method for arrangingsensing portions of a photodetector. FIG. 6A is a diagram showing alightflux area of reflection light (signal light) from a disc. FIG. 6B is adiagram showing a distribution state of signal light on a photodetector(a four-division sensor), in the case where an anamorphic lens based ona conventional astigmatism method and the photodetector are respectivelydisposed at the arrangement position of the anamorphic lens and theplane S0 in the arrangement shown in FIG. 1A. FIGS. 6C and 6D arediagrams showing distribution states of signal light and arrangements ofsensing portions on the plane S0 based on the above principle.

Referring to FIGS. 6A through 6D, a direction of an image (a trackimage) obtained by diffraction of signal light by a track groove isinclined by 45 degrees with respect to the flat surface direction andthe curved surface direction. Referring to FIG. 6A, assuming that thedirection of a track image is aligned with a transverse direction, thedirection of a track image derived from signal light is aligned with avertical direction in FIGS. 6B through 6D. To simplify the description,a light flux is divided into eight light flux areas “a” through “h” inFIGS. 6A and 6B. Further, the track image is shown by the solid line,and the beam shape in an off-focus state is shown by the dotted line. Itis known that a superimposed state of a 0-th order diffraction image anda first order diffraction image of signal light by the track groove isobtained by the ratio: wavelength/(track pitch×NA of an objective lens).As shown in FIGS. 6A, 6B, and 6D, a condition for forming a first orderdiffraction image in the four light flux areas “a”, “d”, “e”, and “h” isexpressed by: wavelength/(track pitch×NA of an objective lens)>√2.

In the conventional astigmatism method, sensing portions P1 through P4of a photodetector (a four-division sensor) are set as shown in FIG. 6B.In this arrangement, a focus error signal FE and a push-pull signal (atracking signal) PP are obtained by implementing the following equations(1) and (2):

FE=(A+B+E+F)−(C+D+G+H)  (1)

PP=(A+B+G+H)−(C+D+E+F)  (2)

where A through H are detection signal components based on lightintensities of light flux areas “a” through “h”.

On the other hand, as described above, signal light is distributed inthe signal light area as shown in FIG. 6C, in the distribution stateshown in FIG. 5B. In this case, signal light passing the light fluxareas “a” through “h” shown in FIG. 6A is as shown in FIG. 6D.Specifically, signal light passing the light flux areas “a” through “h”in FIG. 6A is guided to light flux areas “a” through “h” shown in FIG.6D on the plane S0 where the sensing portions of the photodetector aredisposed.

Accordingly, setting sensing portions P11 through P18 as shown in anoverlapped state in FIG. 6D on the positions corresponding to the lightflux areas “a” through “h” shown in FIG. 6D enables to generate a focuserror signal and a push-pull signal by performing the same computationas the computation described referring to FIG. 6B. Specifically,similarly to the case of FIG. 6B, a focus error signal FE and apush-pull signal PP can be obtained by implementing the equations (1)and (2), wherein A through H are detection signals from the sensingportions for receiving light fluxes in the light flux areas “a” through“h”.

As described above, according to the principle of the embodiment, afocus error signal and a push-pull signal (a tracking error signal),with an influence of stray light being suppressed, can be generated byperforming the same computation as applied in the conventionalastigmatism method.

As already described, the anamorphic lens to be used in the principle ofthe embodiment may have the arrangements as shown in FIGS. 11A and 14A.Specifically, the anamorphic lens to which the principle of theembodiment is applicable, has plural lens areas formed around theoptical axis of laser light, and is configured to impart astigmatism tolaser light individually with respect to each of the lens areas. Theanamorphic lens is configured in such a manner that a light flux isconverged in a direction (e.g. the flat surface direction in FIGS. 1B,11A, and 14A) parallel to one of two boundaries defined by each one ofthe lens areas and the other two lens areas adjacent to the one lensarea around the optical axis of laser light to form a focal line at aposition of a first focal length; and that a light flux is converged inthe direction perpendicular to the boundary to form a focal line at aposition of a second focal length different from the first focal length.

Preferably, the anamorphic lens has four or more lens areas, and theangle of each of the lens areas to be formed around the optical axis isset to 90 degrees or less. The preferred arrangement enables to preventstray light from entering into the signal light area, as shown in e.g.FIG. 5B.

EXAMPLE

In this section, an example of the invention based on the aboveprinciple is described.

FIG. 7 is a diagram showing an optical system of an optical pickupdevice, as an example of the invention. For sake of convenience, arelevant circuit configuration is also shown in FIG. 7. A disc in FIG. 7is formed by laminating plural recording layers.

As shown in FIG. 7, the optical system of the optical pickup deviceincludes a semiconductor laser 101, a polarized beam splitter 102, acollimator lens 103, a lens actuator 104, a rise-up mirror 105, aquarter wavelength plate 106, an aperture 107, an objective lens 108, aholder 109, an objective lens actuator 110, an anamorphic lens 111, anda photodetector 112.

The semiconductor laser 101 emits laser light of a predeterminedwavelength. The divergence angle of laser light to be emitted from thesemiconductor laser 101 is such that a horizontal divergence angle and avertical divergence angle are different from each other.

The polarized beam splitter 102 substantially totally reflects laserlight (S-polarized light) to be entered from the semiconductor laser101, and substantially totally transmits laser light (P-polarized light)to be entered from the collimator lens 103. The collimator lens 103converts laser light to be entered from the polarized beam splitter 102into parallel light.

The lens actuator 104 displaces the collimator lens 103 in an opticalaxis direction in accordance with a servo signal to be inputted from aservo circuit 203. Accordingly, aberration generated in the laser lightis corrected. The rise-up mirror 105 reflects the laser light enteredfrom the collimator lens 103 in a direction toward the objective lens108.

The quarter wavelength plate 106 converts laser light directed to thedisc into circularly polarized light, and converts reflection light fromthe disc into linearly polarized light orthogonal to a polarizationdirection toward the disc. Accordingly, the laser light reflected on thedisc is transmitted through the polarized beam splitter 102.

The aperture 107 adjusts the beam shape of laser light into a circularshape to properly set the effective diameter of laser light with respectto the objective lens 108. The objective lens 108 is designed in such amanner as to properly converge laser light onto a targeted recordinglayer in the disc. The holder 109 integrally holds the quarterwavelength plate 106 and the objective lens 108. The objective lensactuator 110 is constituted of a conventional well-known electromagneticdrive circuit. A coil portion such as a focus coil of theelectromagnetic drive circuit is mounted on the holder 109.

The anamorphic lens 111 imparts astigmatism to reflection light from thedisc. Specifically, the anamorphic lens 111 is configured in such amanner that the curved surface direction and the flat surface directionare defined as described referring to FIG. 1B. Further, the anamorphiclens 111 has a function of changing the propagating direction of laserlight entered from the polarized beam splitter 102 in the manner asdescribed referring to FIG. 5A. The anamorphic lens 111 having the abovefunction is operable to change the propagating directions of lightfluxes passing the areas A through D shown in FIG. 5A, out of the laserlight entered into the anamorphic lens 111, by a certain angle amount α.The angle amount α is defined to such a value that the distributionstate of signal light, and stray light 1 and 2 on the plane S0 becomesthe distribution state shown in FIG. 5B.

The photodetector 112 has the sensing portions P11 through P18 shown inFIG. 6D. The photodetector 112 is arranged at such a position that thesensing portions P11 through P18 are located on the plane S0 shown inFIG. 1A. The sensing portions P11 through P18 of the photodetector 112respectively receive light fluxes passing the light flux areas “a”through “h” shown in FIG. 6D.

A signal computing circuit 201 performs a computation with respect todetection signals outputted from the eight sensing portions of thephotodetector 112 in accordance with the equation (1), and generates afocus error signal. Further, the signal computing circuit 201 generatesa reproduction RF signal by summing up the detection signals outputtedfrom the eight sensing portions. Furthermore, the signal computingcircuit 201 performs a computation with respect to the detection signalsoutputted from the eight sensing portions of the photodetector 112 inaccordance with the equation (2), and generates a push-pull signal PP (atracking error signal). The focus error signal and the push-pull signalPP are transmitted to the servo circuit 203, and the reproduction RFsignal is transmitted to a reproducing circuit 202 and the servo circuit203.

The reproducing circuit 202 demodulates the reproduction RF signalinputted from the signal computing circuit 201, and generatesreproduction data. The servo circuit 203 generates a tracking servosignal and a focus servo signal based on the push-pull signal PP and thefocus error signal inputted from the signal computing circuit 201, andoutputs the tracking servo signal and the focus servo signal to theobjective lens actuator 110. The servo circuit 203 also outputs a servosignal to the lens actuator 104 to optimize the quality of thereproduction RF signal inputted from the signal computing circuit 201. Acontroller 204 controls the respective parts in accordance with aprogram incorporated in an internal memory provided in the controller204.

The signal computing circuit 201 shown in FIG. 7 may be provided in theoptical pickup device or in the optical disc device. Furtheralternatively, a part of the circuit section constituting the signalcomputing circuit 201 may be provided in the optical pickup device, andthe remainder thereof may be provided in the optical disc device.

FIGS. 8A and 8B are diagrams showing an arrangement example of theanamorphic lens 111. FIG. 8A is a perspective view of the anamorphiclens 111, and FIG. 8B is a diagram of the anamorphic lens 111, whenviewed in the optical axis direction of reflection light and from theside of the polarized beam splitter 102.

Referring to FIG. 8A, the anamorphic lens 111 has lens areas 111 athrough 111 d having different curved surface shapes from each other onan incident surface side thereof. The lens areas 111 a through 111 deach has a function of imparting astigmatism to light to be enteredalong an optical axis M, and a function of changing the propagatingdirection of light.

Concerning the astigmatism function, the lens areas 111 a through 111 dare designed in such a manner that a light flux in each one of the lensareas 111 a through 111 d is converged in a direction (the flat surfacedirection) parallel to one boundary Ba1, Bb1, Bc1, and Bd1 of twoboundaries defined between the one lens area, and the other two lensareas adjacent to the one lens area around the optical axis of laserlight to form a focal line at the focal line position (S2) shown in FIG.1A; and that a light flux is converged in a direction (the curvedsurface direction) perpendicular to the boundary Ba1, Bb1, Bc1, and Bd1to form a focal line at the focal line position (S1) different from thefocal line position (S2). The lens areas 11 a through 11 d are alsodesigned in such a manner that the boundary Ba1, Bb1, Bc1, and Bd1parallel to the flat surface direction of each of the lens areas is inproximity to a boundary Ba2, Bb2, Bc2, and Bd2 of the correspondingadjacent lens area, and that these boundaries Ba1, Bb1, Bc1, and Bd1 arenot in proximity to each other.

Concerning the propagating direction changing function, the lens areas11 a through 11 d are designed in such a manner that, as shown in FIG.8B, the propagating directions of laser light to be entered into thelens areas 111 a through 111 d are respectively changed into directionsVa through Vd.

The astigmatism function and the propagating direction changing functionare adjusted in such a manner that laser light (signal light, and straylight 1 and 2) passing the lens areas 111 a through 111 d is distributedon a light receiving surface of the photodetector 112, as shown in FIG.5B, in a state that laser light is focused on a targeted recordinglayer. Accordingly, laser light (signal light, and stray light 1 and 2)to be entered into the lens areas 111 a through 111 d can be properlyreceived on the sensing portions of the photodetector 112. The boundarybetween the corresponding adjacent lens areas is inclined with respectto the direction of a track image by 45 degrees.

As described above, according to the inventive example, it is possibleto prevent signal light reflected on a targeted recording layer out ofthe recording layers in a disc, and stray light 1 and 2 reflected onrecording layers away from and closer to the targeted recording layerfrom superimposing one over the other on the light receiving surface(the plane S0 where the beam spot of signal light becomes a least circleof confusion in an on-focus state) of the photodetector 112.Specifically, it is possible to make a distribution state of signallight, and stray light 1 and 2 on the light receiving surface (the planeS0), as shown in FIG. 5B. Accordingly, arranging the sensing portionsP11 through P18 shown in FIG. 6D at a position corresponding to thesignal light area shown in FIG. 5B enables to receive only thecorresponding signal light on the respective sensing portions P11through P18. This enables to suppress deterioration of a detectionsignal resulting from stray light.

Further, the above effects can be obtained by merely disposing theanamorphic lens 111 shown in FIGS. 8A and 8B on an optical path of laserlight reflected on a disc, specifically, between the polarized beamsplitter 102 and the photodetector 112 in the arrangement shown in FIG.7, in place of using the conventional anamorphic lens. Thus, theinventive example is advantageous in effectively removing an influenceresulting from stray light, with a simplified arrangement.

Further, since the anamorphic lens 111 has both of the astigmatismfunction and the propagating direction changing function, the inventiveexample is advantageous in simplifying the arrangement, as compared witha case that an anamorphic lens only having an astigmatism function andan additional optical element having a propagating direction changingdirection are arranged.

Furthermore, in the inventive example, as shown in FIG. 6C, the signallight area has a square shape, and signal light is irradiated topositions corresponding to vertices of the square. This arrangementenables to make the area for arranging the sensing portions compact, andmakes it easy to arrange the sensing portions.

In the case where the optical pickup device and the optical disc deviceas the inventive example are used in recording or reproducing on or froman optical disc having only two recording layers, the distribution stateof stray light becomes the state as shown in FIGS. 9A and 9B becausethere is only one recording layer other than a targeted recording layer.FIGS. 9A and 9B respectively show distribution states, in the case wherelaser light is focused on a recording layer at a forward position and arecording layer at a rearward position with respect to the targetedrecording layer.

In the above case, as shown in FIGS. 9A and 9B, there is no likelihoodthat stray light may be superimposed one over the other. Accordingly,there is no likelihood that stray light may be interfered with eachother, or stray light amplified by coherence may be leaked into a signallight area, which may result in deterioration of a detection signal.Thus, the inventive example is advantageous in effectively suppressingdeterioration of a detection signal resulting from coherence of straylight, as well as the above effect.

In the case where there is only one recording layer, substantially thesame effect as above can be obtained. Specifically, in the case wherethere is only one recording layer, if a focus position of laser light isdisplaced from the recording layer, reflection light from the disc maybe partly entered into the area of stray light shown in FIGS. 9A and 9B.In this case, however, there is no likelihood that reflection lightpartly entered into the area of stray light may be superimposed one overthe other. Accordingly, there is no likelihood that reflection lightpartly entered into the area of stray light may be interfered with eachother. Thus, the above arrangement enables to generate a high-qualitysignal from the sensing portions arranged in the signal light area.

The stray light removal effect based on the above principle can beobtained, in the case where the focal line position (M12) of stray light1 in the flat surface direction is closer to the anamorphic lens 111than the plane S0, and the focal line position (M21) of stray light 2 inthe curved surface direction is away from the anamorphic lens 111 thanthe plane S0, referring to FIG. 1A. Specifically, as far as the aboverelation is satisfied, the distribution state of signal light, and straylight 1 and 2 becomes the state as shown in FIG. 5A, which makes itpossible to prevent signal light, and stray light 1 and 2 from beingsuperimposed one over the other on the plane S0. In other words, as faras the above relation is satisfied, even if the focal line position(M12) of stray light 1 in the flat surface direction comes closer to theplane S0 than the focal line position (S1) of signal light in the curvedsurface direction, or the focal line position (M21) of stray light 2 inthe curved surface direction comes closer to the plane S0 than the focalline position (S2) of signal light in the flat surface direction, theeffects of the invention and the inventive example based on the aboveprinciple can be obtained.

Method for Adjusting Positions of Photodetector and Anamorphic Lens

In the inventive example, it is necessary to adjust the positions of theanamorphic lens 111 and the photodetector 112 to such positions thatsignal light, out of light fluxes (signal light, and stray light 1 and2) passing the lens areas 111 a through 111 d shown in FIG. 8A isproperly entered into the sensing portions P11 through P18 shown in FIG.6D arranged on the photodetector 112. The positional adjustment can beperformed by e.g. the following technique.

FIGS. 10A through 10D are diagrams showing a method for adjusting thepositions of the anamorphic lens 111 and the photodetector 112.

The anamorphic lens 111 is configured in such a manner that the lensareas 111 a through 111 d have the dimensions large enough to receivereflection light fluxes from a disc in the vicinity of the arrangedposition of the anamorphic lens 111. Further, the photodetector 112 isconfigured in such a manner that sensing portions P11′, P12′, P13′,P15′, P17′, and P18′ are provided above the sensing portions P14 and P16so that the positional adjustment of the photodetector 112 based on theconventional astigmatism method can be performed.

In adjusting the positions of the anamorphic lens 111 and thephotodetector 112, first, as shown in FIG. 10A, the position of theanamorphic lens 111 is roughly adjusted so that a reflection light fluxfrom a disc generally lies in the lens area 111 c, and the anamorphiclens 111 is provisionally fixed thereat.

The reflection light flux entered into the lens area 111 c from the discis subjected to astigmatism by the lens area 111 c, and the propagatingdirection of the reflection light flux is changed at the lens area 111c, without being separated by the anamorphic lens 111. Accordingly, thereflection light flux transmitted through the lens area 111 c forms sucha beam spot based on astigmatism that the reflection light flux has acircular shape in an on-focus state and an elliptical shape in anoff-focus state on the light receiving surface of the photodetector 112.

Next, the position of the photodetector 112 is adjusted by an adjustmentmethod based on the conventional astigmatism method by using outputsignals from the sensing portions P14, P16, P11′, P12′, P13′, P15′,P17′, and P18′ in such a manner that the beam spot is properly detectedby the photodetector 112, as shown in FIG. 10B; and the photodetector112 is fixed at a proper position. In this case, the position of thephotodetector 112 is adjusted, based on detection signals from foursensing areas divided by two parting lines L1 and L2. Accordingly, thecentroid of the light flux can be aligned with a targeted position ofthe photodetector 112.

After the position of the photodetector 112 is determined, the positionof the anamorphic lens 111, whose position has been provisionally fixed,is adjusted. Referring to FIGS. 10C and 10D, the anamorphic lens 111 ismoved on a plane perpendicular to the optical axis, and fixed at such aposition that signals to be detected from the sensing portions P11through P18 of the photodetector 112 become equal to each other. Byperforming the above operation, the crossing point of the parting linesof the anamorphic lens 111, and the centroid of the reflection lightflux can be precisely aligned with each other. Thus, both of theanamorphic lens 111 and the photodetector 112 can be fixed at therespective proper positions.

Modification Example 1

In the inventive example, light fluxes of signal light, and stray light1 and 2 have the distribution state as shown in FIG. 5B on the plane S0by the astigmatism function and the propagating direction changingfunction shown in FIG. 5A, and the light fluxes are received on thesensing portions shown in FIG. 6D. In modification example 1, it ispossible to prevent light fluxes of signal light, and stray light 1 and2 from being superimposed one over the other on the plane S0 by anastigmatism function and a propagating direction changing functiondifferent from those shown in FIG. 5A.

FIG. 11A is a diagram showing an arrangement of an anamorphic lens inmodification example 1. The anamorphic lens in modification example 1 isdivided into six areas around an optical axis. The areas A and D have anangle of 90 degrees in the circumferential direction of the opticalaxis, and the areas B, C, E, and F have an angle of 45 degrees in thecircumferential direction of the optical axis. Each of the areas has anastigmatism function and a propagating direction changing function inthe similar manner as the inventive example.

Concerning the astigmatism function, the areas A through F are designedin such a manner that a light flux is converged in a direction (a flatsurface direction) parallel to one of two boundaries defined by each oneof the areas A through F, and the other two areas adjacent to the onearea around the optical axis of laser light to form a focal line at thefocal line position (S2) shown in FIG. 1A; and that a light flux isconverged in a direction perpendicular to the one boundary to form afocal line at the focal line position (S1) different from the focal lineposition (S2).

Concerning the propagating direction changing function, the areas Athrough F are designed in such a manner that the propagating directionsof laser light to be entered into the areas A through F are respectivelychanged into directions Da through Df shown in FIG. 11A. The directionsDa and Db are respectively aligned with the Z-axis positive directionand the Z-axis negative direction, and the directions Db, Dc, De, and Dfare each aligned with a direction inclined with respect to Y-axis by67.5 degrees, and inclined with respect to Z-axis by 22.5 degrees.

The astigmatism function and the propagating direction changing functionare adjusted in such a manner that laser light (signal light, and straylight 1 and 2) passing the areas A through F is distributed as shown inFIG. 11B on the light receiving surface of the photodetector 112 shownin FIG. 7 in a state that the laser light is focused on a targetedrecording layer. Accordingly, similarly to the inventive example, it ispossible to define a signal light area where only signal light exists,and only signal light can be received by the respective correspondingsensing portions by arranging the sensing portions at a positioncorresponding to the signal light area.

FIGS. 12A and 12B are diagrams for describing a method for arranging thesensing portions in modification example 1. FIG. 12A is a diagramshowing light flux areas “a” through “f” corresponding to reflectionlight (signal light) from a disc, which are entered into the areas Athrough F of the anamorphic lens shown in FIG. 11A. FIG. 12B is adiagram showing the sensing portions based on modification example 1.

Referring to FIG. 12B, each of light fluxes (signal light) in the lightflux areas “a” through “f” is received by two corresponding sensingportions. In FIG. 12B, symbols a1 and a2, b1 and b2, c1 and c2, d1 andd2, e1 and e2, and f1 and f2 respectively denote light fluxes obtainedby dividing the light fluxes in the light flux areas “a” through “f”into two. As shown in FIG. 12B, the light fluxes a1 through f2 arerespectively received by corresponding sensing portions P21 through P32.Specifically, the sensing portions P21 and P22, the sensing portions P23and P24, the sensing portions P25 and P26, the sensing portions P27 andP28, the sensing portions P29 and P30, and the sensing portions P31 andP32 are set at such positions that a half of the respective light fluxesin the light flux areas “a” through “f” is received at a time whensignal light is focused on a targeted recording layer.

In modification example 1, assuming that detection signals based on thelight receiving amounts of the sensing portions P21 through P32 arerespectively A1, A2, B1, B2, C1, C2, D1, D2, E1, E2, F1, and F2, apush-pull signal PP is obtained by performing the following computation:

PP=(A1+A2+B1+B2+F1+F2)−(D1+D2+C1+C2+E1+E2)

FIGS. 13A through 13C are diagrams showing detection signals and a focuserror signal from each of the sensing portions. In FIGS. 13A through13C, the axis of ordinate denotes a detection signal from a sensingportion, or a computation result, and the axis of abscissas denotes adistance between an objective lens and a disc. The original point of theaxis of abscissas denotes a position of the objective lens where signallight is focused on a targeted recording layer (hereinafter, called as a“focus position”).

Referring to FIG. 13A, a symbol s1 denotes the detection signals A1 andD1, and a symbol s2 denotes the detection signals A2 and D2. In thiscase, a symbol s3 representing (s1-s2) becomes zero at a focus position,as shown in FIG. 13A.

Referring to FIG. 13B, a symbol s4 denotes the detection signal B1, C1,E1, F1, and a symbol s5 denotes the detection signal B2, C2, E2, F2. Inthis case, a symbol s6 representing (s4-s5) becomes zero at a focusposition, as shown in FIG. 13B.

Accordingly, the focus error signal FE can be expressed by using A1through F2 as follows:

FE={(A1−A2)+(D1−D2)+{(B1−B2)+(C1−C2)+(E1−E2)+(F1−F2)}

FIG. 13C is a diagram showing the focus error signal FE. As shown inFIG. 13C, the focus error signal has an S-shaped curve, and becomes zeroat a focus position. Accordingly, the focus position of signal light canbe adjusted on a targeted recording layer by driving the objective lensin a direction perpendicular to the optical axis to such a position thatthe focus error signal FE becomes zero.

As described above, similarly to the inventive example, in modificationexample 1, a signal light area can be defined by the anamorphic lensshown in FIG. 11A, and a high-quality detection signal, with aninfluence by stray light being suppressed, can be obtained by arrangingthe photodetector having the sensing portions as shown in FIG. 12B at aposition corresponding to the signal light area.

Modification Example 2

In modification example 2, it is possible to prevent signal light, andstray light 1 and 2 from being superimposed one over the other by usingan anamorphic lens different from the anamorphic lens shown in FIG. 11A.

FIG. 14A is a diagram showing an arrangement of an anamorphic lens inmodification example 2. The anamorphic lens in modification example 2 isdivided into eight areas around an optical axis. Each of the areas hasan angle of 45 degrees in the circumferential direction of the opticalaxis, and has an astigmatism function and a propagating directionchanging function in the similar manner as the inventive example.

Concerning the astigmatism function, the areas A through H are designedin such a manner that a light flux is converged in a direction (a flatsurface direction) parallel to one of two boundaries defined by each oneof the areas A through H, and the other two areas adjacent to the onearea around the optical axis of laser light to form a focal line at thefocal line position (S2) shown in FIG. 1A; and that a light flux isconverged in a direction perpendicular to the one boundary to formafocal line at the focal line position (S1) different from the focal lineposition (S2).

Concerning the propagating direction changing function, the areas Athrough H are designed in such a manner that the propagating directionsof laser light to be entered into the areas A through H are respectivelychanged into directions Da through Dh shown in FIG. 14A. The directionsDa through Dh are inclined with respect to the flat surface directionsof the respective areas by 67.5 degrees.

The astigmatism function and the propagating direction changing functionare adjusted in such a manner that laser light (signal light, and straylight 1 and 2) passing the areas A through H is distributed as shown inFIG. 14B on the light receiving surface of the photodetector 112 shownin FIG. 7 in a state that the laser light is focused on a targetedrecording layer. Accordingly, similarly to the inventive example, it ispossible to define a signal light area where only signal light exists,and only signal light can be received by the respective correspondingsensing portions by arranging the sensing portions at a positioncorresponding to the signal light area.

FIGS. 15A and 15B are diagrams for describing a method for arranging thesensing portions in modification example 2. FIG. 15A is a diagramshowing light flux areas “a” through “h” corresponding to reflectionlight (signal light) from a disc, which are entered into the areas Athrough H of the anamorphic lens shown in FIG. 14A. FIG. 15B is adiagram showing the sensing portions based on modification example 2.

Referring to FIG. 15B, each of light fluxes (signal light) of the lightflux areas “a” through “h” is received by two corresponding sensingportions. In FIG. 15B, the symbols a1 and a2, b1 and b2, c1 and c2, d1and d2, e1 and e2, f1 and f2, g1 and g2, and h1 and h2 respectivelydenote light fluxes obtained by dividing the light fluxes in the lightflux areas “a” through “h” into two. As shown in FIG. 15B, the lightfluxes a1 through h2 are respectively received by corresponding sensingportions P41 through P56. Specifically, the sensing portions P41 andP42, the sensing portions P43 and P44, the sensing portions P45 and P46,the sensing portions P47 and P48, the sensing portions P49 and P50, thesensing portions P51 and P52, the sensing portions P53 and P54, and thesensing portions P55 and P56 are designed in such a manner that a halfof the respective light fluxes in the light flux areas “a” through “h”is received at a time when signal light is focused on a targetedrecording layer.

In modification example 2, assuming that detection signals based on thelight receiving amounts of the sensing portions P41 through P56 arerespectively A1, A2, B1, B2, C1, C2, D1, D2, E1, E2, F1, F2, G1, G2, H1,and H2, a push-pull signal PP is obtained by performing the followingcomputation:

PP=(A1+A2+B1+B2+G1+G2+H1+H2)−(D1+D2+E1+E2+C1+C2+F1+F2)

FIGS. 16A and 16B are diagrams showing a detection signal from each ofthe sensing portions, and a focus error signal.

Referring to FIG. 16A, a symbol s7 denotes the detection signal A1, B1,C1, D1, E1, F1, G1, H1, and a symbol s8 denotes the detection signal A2,B2, C2, D2, E2, F2, G2, H2. In this case, a symbol s9 representing(s7-s8) becomes zero at a focus position, as shown in FIG. 16A.

Accordingly, the focus error signal FE can be expressed by using A1through H2 as follows:

FE=(A1−A2)+(B1−B2)+(C1−C2)+(D1−D2)+(E1−E2)+(F1−F2)+(G1−G2)+(H1−H2)

FIG. 16B is a diagram showing the focus error signal FE. As shown inFIG. 16B, the focus error signal has an S-shaped curve, and becomes zeroat a focus position. Accordingly, the focus position of signal light canbe arranged on a targeted recording layer by driving the objective lensto such a position that the focus error signal FE becomes zero.

As described above, similarly to the inventive example, in modificationexample 2, a signal light area can be defined by the anamorphic lensshown in FIG. 14A, and a high-quality detection signal, with aninfluence by stray light being suppressed, can be obtained by arrangingthe photodetector having the sensing portions as shown in FIG. 15B at aposition corresponding to the signal light area.

Other Modifications

In the foregoing description, the inventive example, and modificationexamples 1 and 2 are described. The invention is not limited to theforegoing examples, and the embodiment of the invention may be modifiedin various ways other than the above.

For instance, in the inventive example, the anamorphic lens 111 isprovided with both of the astigmatism function shown in FIG. 1A and thepropagating direction changing function shown in FIG. 5A. Alternatively,the anamorphic lens 111 may be provided with only the astigmatismfunction, and another optical element may be provided with thepropagating direction changing function. Further alternatively, ahologram element may be provided with both of the functions. In thelatter modification, the hologram element may be used, in place of theanamorphic lens 111. A hologram pattern of the hologram element may bedesigned by a well-known technique.

FIGS. 17A through 17C are diagrams showing a direction changing element120 for imparting a propagating direction changing function. FIG. 17Ashows an arrangement example wherein the direction changing element 120is constituted of a hologram element having a diffraction pattern. FIGS.17B and 17C show an arrangement example, wherein the direction changingelement 120 is constituted of a multifaceted prism.

In the arrangement example shown in FIG. 17A, the direction changingelement 120 is formed of a transparent plate of a square shape, and ahologram pattern is formed on a light incident surface of the directionchanging element 120. As shown in FIG. 17A, the light incident surfaceis divided into four hologram areas 120 a through 120 d. Further,similarly to FIG. 5A, the hologram areas 120 a through 120 d areoperable to diffract incident laser light (signal light, and stray light1 and 2) in directions Va through Vd, respectively. With the abovearrangement, substantially the same effect as the anamorphic lens 111 inthe inventive example can be obtained by the anamorphic lens and thedirection changing element 120. The hologram to be formed on thehologram areas 120 a through 120 d may have a stepped pattern or ablazed pattern.

Further alternatively, the arrangement shown in FIG. 17B may be used inplace of the arrangement shown in FIG. 17A. In the modification shown inFIG. 17B, the direction changing element 120 is formed of a transparentmember provided with a flat light exit surface, and a light incidentsurface having four areas which are inclined in different directionsfrom each other.

FIG. 17C is a diagram of the direction changing element 120 shown inFIG. 17B, when viewed from the light incident surface side. As shown inFIG. 17C, four slopes 120 e through 120 h are formed on the lightincident surface of the direction changing element 120. Upon incidenceof a light ray parallel to X-axis from the incident surface side intothe slopes 120 e through 120 e, the propagating directions of light arerespectively changed into directions Va through Vd shown in FIG. 17C byrefraction of light at the time of incidence into the slopes 120 ethrough 120 h. In the modification, substantially the same effect as theeffect to be obtained by the anamorphic lens 111 in the inventiveexample can be obtained by the anamorphic lens and the directionchanging element 120.

In the case where the focal line positions generated by an astigmatismfunction have the positional relation as shown in FIG. 1A, the directionchanging element 120 is disposed at a position closer to the disc thanthe focal line position (M11) of stray light 1 in the curved surfacedirection. In this arrangement, the distribution state of stray light 1and 2 on the plane S0 (the light receiving surface of the photodetector112) becomes the state as shown in FIGS. 9A and 9B, as described in thedescription on the principle and the inventive example.

Further alternatively, the direction changing element 120 may bedisposed between the focal line position (M12) and the focal lineposition (S1) shown in FIG. 1A. In the modification, similarly to theabove, the distribution state of stray light 2 on the plane S0 (thelight receiving surface of the photodetector 112) becomes the state asshown in FIG. 9B, and likewise, the distribution state of stray light 1also becomes the state as shown in FIG. 9B. Since the direction changingelement is arranged at a position closer to the photodetector, it ispossible to integrally form the direction changing element and thephotodetector.

In modification examples 1 and 2, it is possible to provide theanamorphic lens 111 only with the astigmatism function, and provideanother optical element with the propagating direction changingfunction. In the modification, similarly to the arrangements shown inFIGS. 17A through 17C, an optical element for imparting the propagatingdirection changing function may be constituted of a hologram element ora light refraction element.

In the inventive example, and modification examples 1 and 2, it ispossible to use a polarization converting element which convertspolarization directions of light in adjacent light flux areas indirections orthogonal to each other. In the modification, thepolarization converting element is disposed at a position closer to thedisc than the optical element having a propagating direction changingelement. Specifically, the polarization converting element is disposedat a position closer to the disc than the anamorphic lens 111 in theinventive example, and the direction changing element 120 in the othermodification.

In the above modification, the polarization directions are aligned indirections orthogonal to each other even on a portion where the lightflux areas of stray light 1 and 2 are superimposed one over the other onthe photodetector. Accordingly, coherence of stray light on thesuperimposed portion can be suppressed. Therefore, amplification of astray light component resulting from coherence of stray light can besuppressed, which enables to suppress leakage of stray light into asensing portion.

The embodiment of the invention may be changed or modified in variousways as necessary, as far as such changes and modifications do notdepart from the scope of the claims of the invention hereinafterdefined.

1. An optical pickup device for irradiating laser light onto a recordingmedium having a plurality of recording layers, comprising: a lightsource for emitting laser light; an objective lens for converging thelaser light on the recording medium; an astigmatism portion forreceiving the laser light reflected on the recording medium, andprovided with a plurality of lens areas formed around an optical axis ofthe laser light to impart astigmatism to the laser light individuallywith respect to each of the lens areas; a light flux separating portionfor making propagating directions of a light flux of the laser light tobe entered into each of the lens areas different from each other toseparate the light fluxes from each other; and a photodetector forreceiving the separated light fluxes to output a detection signal. 2.The optical pickup device according to claim 1, wherein the lens areasare configured in such a manner that the light flux in each one of thelens areas is converged in a direction parallel to a first boundary outof the first boundary and a second boundary defined between the one lensarea, and the other two lens areas adjacent to the one lens area aroundthe optical axis to form a focal line at a position of a first focallength, and that the light flux is converged in a directionperpendicular to the first boundary to form a focal line at a positionof a second focal length different from the first focal length, and thefirst boundaries of the respective lens areas are defined at suchpositions that the first boundaries are not in proximity to each other.3. The optical pickup device according to claim 2, wherein the lightflux separating portion is configured in such a manner that, in the casewhere the laser light is focused on a targeted recording layer of therecording medium, the light fluxes of the laser light reflected on thetargeted recording layer, and the light fluxes of the laser lightreflected on a recording layer other than the targeted recording layerare not superimposed one over the other on a light receiving surface ofthe photodetector.
 4. The optical pickup device according to claim 1,wherein the astigmatism portion and the light flux separating portionare integrally formed into an optical element.
 5. The optical pickupdevice according to claim 1, wherein the astigmatism portion has four ormore of the lens areas, and an angle of each of the lens areas aroundthe optical axis is set to 90 degrees or less.
 6. The optical pickupdevice according to claim 1, wherein the astigmatism portion has fourlens areas, the angle of each of the lens areas around the optical axisis set to 90 degrees, and the light flux separating portion isconfigured in such a manner that, in the case where the laser light isfocused on a targeted recording layer of the recording medium, each ofthe light fluxes of the laser light reflected on the targeted recordinglayer is positioned at a position corresponding to a vertex of a squareon a light receiving surface of the photodetector.
 7. An optical discdevice, comprising: an optical pickup device; and a computing circuit,the optical pickup device including a light source for emitting laserlight; an objective lens for converging the laser light on the recordingmedium; an astigmatism portion for receiving the laser light reflectedon the recording medium, and which is provided with a plurality of lensareas formed around an optical axis of the laser light to impartastigmatism to the laser light individually with respect to each of thelens areas; a light flux separating portion for making propagatingdirections of a light flux of the laser light to be entered into each ofthe lens areas different from each other to separate the light fluxesfrom each other; and a photodetector for receiving the separated lightfluxes to output a detection signal, wherein the computing circuitprocesses an output from the photodetector.
 8. The optical disc deviceaccording to claim 7, wherein the lens areas are configured in such amanner that the light flux in each one of the lens areas is converged ina direction parallel to a first boundary out of the first boundary and asecond boundary defined between the one lens area, and the other twolens areas adjacent to the one lens area around the optical axis to forma focal line at a position of a first focal length, and that the lightflux is converged in a direction perpendicular to the first boundary toform a focal line at a position of a second focal length different fromthe first focal length, and the first boundaries of the respective lensareas are defined at such positions that the first boundaries are not inproximity to each other.
 9. The optical disc device according to claim8, wherein the light flux separating portion is configured in such amanner that, in the case where the laser light is focused on a targetedrecording layer of the recording medium, the light fluxes of the laserlight reflected on the targeted recording layer, and the light fluxes ofthe laser light reflected on a recording layer other than the targetedrecording layer are not superimposed one over the other on a lightreceiving surface of the photodetector.
 10. The optical disc deviceaccording to claim 7, wherein the astigmatism portion and the light fluxseparating portion are integrally formed into an optical element. 11.The optical disc device according to claim 7, wherein the astigmatismportion has four or more of the lens areas, and an angle of each of thelens areas around the optical axis is set to 90 degrees or less.
 12. Theoptical disc device according to claim 7, wherein the astigmatismportion has four lens areas, the angle of each of the lens areas aroundthe optical axis is set to 90 degrees, and the light flux separatingportion is configured in such a manner that, in the case where the laserlight is focused on a targeted recording layer of the recording medium,each of the light fluxes of the laser light reflected on the targetedrecording layer is positioned at a position corresponding to a vertex ofa square on a light receiving surface of the photodetector.